A separative roll-type mechanism installed between an automatic fish selector and counting system was devised. The separative state of overlapping fish body was investigated by controlling the peripheral velocity of the first roll on the supply side, and the ratio of peripheral velocities of neighboring rolls, by adjusting the angle of inclination of the line connecting the centers of rolls parallel to each other and the interval distance between neighbor rolls, by controlling the ratio of overlapping fishes body length, and from the relationship between the distance of partitive boards and fish body width. The results were as follows.
1. When the ratio of peripheral velocity of neighboring the rolls was 1.00, the peripheral velocity of the first roll was 0.8m/s, and the ratio of overlapping fishes body length was 75%, overlapping fry were perfectly separated.
2. When the ratio of peripheral velocity of neighboring rolls was 1.26 and the ratio of overlapping fishes body length was 75%, the separation of overlapping fishes body was not affected by the peripheral velocity of the first roll on the supply side, and the counting error of fry numbers was zero.